Cables having large numbers of insulated conductors are widely used today, for example, in telephone communication systems. Frequently these cables are either buried underground or suspended above ground between poles or towers. Faults sometimes develop between two or more of the conductors of these cables, requiring repair or replacement of the cable or a portion thereof. Such faults typically include a short circuit between two or more of the conductors, or a partial breakdown of the insulation between the conductors and high resistance connections caused by water or moisture within the cable. To facilitate the correction of a fault, it is desirable to know the exact location of the fault. This is particularly true in the case of underground cables where a hole must generally be dug to permit access to the troubled area.
Access points are generally provided at pedestals or towers located at spaced apart positions along underground cables and aerial cables. Heretofore, there have been some attempts to determine the location of a fault between the conductors of a cable by means of measurements made on the cable at one of the access points. In one widely used technique of the prior art, a bridge circuit, such as a Varley bridge, is used to determine the resistance between the access point and the fault. From the value of resistance measured, the lineman or repairman calculates the length of the conductor. This approach has several disadvantages. For example, it requires calculations on the part of the lineman or repairman which are time consuming and subject to human error. Resolution drops off toward the ends of the cable, making it difficult to locate faults accurately in these areas. Sensitivity is relatively poor, and is not possible to locate some high resistance faults with the bridge approach. Also, bridges typically pass currents on the order of hundreds or thousands of microamperes through the faulty conductor, and currents of this level sometimes dry out or temporarily eliminate the fault, thereby preventing its location and correction.
A test circuit disclosed in U.S. Pat. No. 3,800,216 to Robert H. Hamilton avoids some of the disadvantages associated with the prior art Varley bridge in that his test circuit employs current levels that are very low and uses an adjustable control having a dial calibrated in units of length which eliminates the need for the operator to make complicated calculations. Use of such test circuit, however, requires a selection to be made from a plurality of calibration resistors plus the adjustment of a number of potentiometer settings. Such operator functions provide chances for operator errors and is also time consuming.